Field of the Invention
The present invention relates to a current source that can be switched between a current-controlled mode and a voltage-controlled mode.
Such current sources are used in a multitude of applications, for example in connection with digital/analog converters.
In the current-controlled mode, the switchable current source is supplied with a current Iref which passes on to other circuit sections on an integrated circuit via a current mirror. In the voltage-controlled mode, the switchable current source is supplied with a reference voltage Vref which converts it to a reference current Iref=Vref/R via a resistor R, and this current, too, is passed on to the other circuit sections of the integrated circuit via the current mirror.
These two modes are required particularly frequently in video chips for the consumer market in order to provide manufacturers of television sets with a greater flexibility in the basic setting of the picture information outputs for the RGB signals.
A circuit configuration suitable for this purpose is described in U.S. Pat. No. 4,814,688. That circuit configuration has the disadvantage, however, that there an operational amplifier no longer has negative feedback in the current-controlled mode which can trigger fluctuations in its output and, as a result, substrate disturbances which can lead to picture faults.
That disadvantage is eliminated by the circuit configuration described in U.S. Pat. No. 5,142,219. This is done by the operational amplifier being switched off in the current-controlled mode with the aid of a comparator and a circuit called a xe2x80x9cpower-downxe2x80x9d circuit.
A significant disadvantage of the circuit configuration known from U.S. Pat. No. 5,142,219 consists, as can be seen from FIGS. 5 and 6 of that document, in the necessity of having to provide four package pinouts of an integrated circuit for the range of functions required. As a result, the switchable current source becomes larger and more expensive.
It is accordingly an object of the invention to provide a switchable current source, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which provides for a current source which can be switched between a current-controlled mode and a voltage-controlled mode, which operates in a stable manner under all circumstances and which manages with a minimum number of package pinouts.
With the foregoing and other objects in view there is provided, in accordance with the invention, a current source that can be switched between a current-controlled mode and a voltage-controlled mode. The circuit comprises:
first and second input connections and one output connection;
a transistor having a drain terminal connected to the first input connection and having a gate or base connected to the output connection;
an operational amplifier having a first input terminal connected to the first input connection and a second input terminal connected to the second input connection;
controllable switches connected to ensure that the transistor is driven by the output signal of the operational amplifier in the voltage-controlled mode and is driven by the voltage at the drain terminal of the transistor in the current-controlled mode;
a comparator generating an output signal for controlling the controllable switches;
the comparator having an input terminal connected to the second input connection; and
wherein a potential to be applied to the second input connection in the current-controlled mode clearly differs from a occurring at the second input connection in the voltage-controlled mode and from a potential occurring at the first input connection in the current-controlled mode.
In accordance with an added feature of the invention, when the current source is to be operated in the current-controlled mode:
the first input connection is supplied with a reference current; and
the second input connection is placed at a potential by way of which the output signal of the comparator is brought to a value by way of which the switches are switched to a position the switches must assume in the current-controlled mode.
In accordance with an additional feature of the invention, when the current source is to be operated in the voltage-controlled mode:
the first input connection is connected to a reference potential via a resistor; and
a reference voltage is applied to the second input connection.
In accordance with another feature of the invention, the potential applied to the second input connection in the current-controlled mode is selected such that a threshold voltage of the comparator lies between the potential applied to the second input connection and a potential at the second input connection in the voltage-controlled mode.
In accordance with a further feature of the invention, the threshold voltage of the comparator is selected to lie in a vicinity of a potential to be applied to the second input connection in the current-controlled mode.
In accordance with again an added feature of the invention, the threshold voltage of the comparator is selected to lie in a vicinity of a potential of one pole of a supply voltage.
In accordance with again an additional feature of the invention, the threshold voltage of the comparator is selected to lie closer to a potential of one pole of the supply voltage than to a potential of the other pole of the supply voltage.
In accordance with again another feature of the invention, the threshold voltage of the comparator is selected to lie 100 to 1500 mV above the potential of the pole of the supply voltage exhibiting the lower potential.
In accordance with again a further feature of the invention, the threshold voltage of the comparator is selected to lie 100 to 1500 mV below the potential of the pole of the supply voltage exhibiting the higher potential.
In accordance with a concomitant feature of the invention, the potential to be applied to the second input connection in the current-controlled mode, and the potential occurring at the first input connection in the current-controlled mode, are selected such that a difference therebetween is so great that the operational amplifier is overdriven.
The current source according to the invention is distinguished by the fact that it is provided with only two input connections and one output connection,
the output connection being connected to the gate or base terminal of a transistor,
the current source having controllable switches which ensure that the transistor is driven by the output signal of an operational amplifier in the voltage-controlled mode and is driven by the voltage occurring at the drain terminal of the transistor in the current-controlled mode,
the switches being controlled in dependence on the output signal of a comparator,
the first input connection of the current source being connected to the drain terminal of the transistor and a first input terminal of the operational amplifier,
the second input connection of the current source being connected to the second input terminal of the operational amplifier and one of the input terminals of the comparator, and
the potential to be applied to the second input connection of the current source in the current-controlled mode clearly differing both from the potential occurring at the second input connection of the current source in the voltage-controlled mode and from the potential occurring at the first input connection of the current source in the current-controlled mode.
The fact that the current source manages with only two input connections and one output connection is caused, in particular, by the particular choice of voltages applied to the input connections or, respectively, the impressed currents.
Due to the fact that the potential applied to the second input connection of the current source in the current-controlled mode is selected in such a manner that it clearly differs from the potential applied to this input connection in the voltage-controlled mode, the switching threshold of the comparator can be set to a value which ensures under all circumstances that the switches controlled by the comparator are operated in such a manner that the current source operates in the current-controlled mode when it is intended to operate in the current-controlled mode and that the current source operates in the voltage-controlled mode when it is intended to operate in the voltage-controlled mode.
Due to the fact that the potential applied to the second input connection of the current source in the current-controlled mode is selected in such a manner that it clearly differs from the potential present at the first input connection of the current source in the current-controlled mode, a large voltage difference or, more precisely, a voltage difference overdriving the operational amplifier, is obtained at the input terminals of the operational amplifier as a result of which the operational amplifier always outputs the maximum output signal and the output signal, therefore, is not subject to any, or, at the most, negligibly small fluctuations.
This results in a reliable switchover resistant to disturbances, on the one hand, and, on the other hand, in an output current which is stable under all circumstances, even though the current source only has two input connections.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a switchable current source, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.